The invention relates to an electrically controlled unit fuel injector as defined hereinafter.
A unit fuel injector of this kind for Diesel engines, known from U.S. Pat. No. 4,669,659, is installed directly in the cylinder head of an associated engine and includes both a mechanically driven piston injection pump and an associated injection nozzle in a common housing. In this unit fuel injector, the fuel injection quantity positively displaced from the pump work chamber to the injection nozzle at the pumping pressure of the pump piston is determined by how long an electromagnetically actuated control valve that is open when it is without current is on; the control valve is located in an overflow conduit that connects the pump work chamber to a low-pressure chamber. When the control valve, in order to control the fuel injection, blocks the communication between the two segments of the overflow conduit and thus blocks the outflow of the fuel to the low-pressure chamber, the first segment of the overflow conduit, which is in continuous communication with the pump work chamber, is then subject to the full injection pressure.
In rare cases it can happen that the control valve will jam; in other words, it becomes stuck in this switching position. The possibility then exists that by reaspiration of fuel by the pump piston, fuel will continue to be delivered into the pump work chamber. This can for instance happen if a check valve that is intended to decouple the pump work chamber from the tank fails to close completely, or if a throttle in the return line stays open. If the injection event continues unintentionally in this way, there is the danger that the vehicle driven by the engine will go out of control. This must be prevented.